In a general,conventional planographic printing machine, the amount of moisturing water supplied to the cylinder has been controlled according to the non-printing area that needs the water most. Since the same amount of water is supplied to the printing area that does not need it, the superfluous water is forced into the ink supply system via the returning portion of the periphery of the inking roll.
However, the conventional ink supply system does not suffer from serious disadvantages, because the ink once taken out of an ink supply source is not returned to the source, and because the inking train consists of as many as about a dozen of rolls, substantially eliminating the possibility that the superfluous water intrudes into the supply source.
Recently, as shown in FIG. 1, a meshed roll 3 has been introduced in a planographic printing system. This roll can dispense with a number of inking roll, but poses problems as described below. The meshed roll 3 is connected to a cylinder 5 via only one pair of inking rolls, 4 and 4a. The superfluous water 7 supplied from a moisturing water supply system 6 to the cylinder 5 flows along the returning portion of the periphery of the inking roll 4, and then intrudingly arrives on the returning portion of the periphery of the meshed roll 3. Subsequently, the water is conveyed to the returning portion 8a of the periphery of a fountain roll 8 and added to the ink stored in an ink pan 9.
The meshed roll 3 is supplied with ink in a peculiar fashion. Specifically, all the ink supplied from the fountain roll 8 to the periphery of the meshed roll 3 is always scraped off by a doctor blade 10 except for the ink contained in the meshes of the roll 3. The scraped superfluous ink is returned into the pan 9, which also contains water. Thereafter, the ink adheres to the periphery of the fountain roll 8, and is again supplied to the roll 3. If water is introduced into the resupplied ink, then the nature of the ink varies, lowering the quality of the printed paper. In addition, after ink is transferred out of the meshed roll 3, water intrudes into the meshes. Thus, ink is less likely to enter the meshes. If water becomes superfluous locally on the roll, then ink is not transferred to the roll. This phenomenon is known as stripping.